This invention is generally directed to liquid developer compositions and, more specifically, the present invention relates to a liquid developer containing certain charge director mixtures. More specifically, the present invention relates to liquid developers comprised of charge directors comprised of mixtures wherein the mixtures can contain in embodiments from about 1 to about 99 weight percent of diesters, and from about 99 weight percent to about 1 weight percent of the aluminum complex mixture of organic phosphate mono and diesters and organic aluminum complexes of the following formulas ##STR2## wherein R.sub.1 is selected from the group consisting of hydrogen and alkyl; wherein alkyl, for example, contains from 1 to about 12 carbon atoms, and n represents a number, such as 1, 2, 3, or 4; and wherein the preferred aluminum complex embodiments is an aluminum-di-tertiary-butyl salicylate, or ALOHAS. The developers of the present invention can be selected for a number of known imaging systems, such as xerographic imaging and printing processes, including charged area development wherein latent images are rendered visible with the liquid developers illustrated herein. For image quality, solid area coverage and resolution of developed images one usually desires, for example, sufficient toner particle electrophoretic mobility. The mobility for effective image development is primarily dependent on the imaging system used, and this electrophoretic mobility is directly proportional to the charge on the toner particles and inversely proportional to the viscosity of the liquid developer fluid. For example, an about 10 to 30 percent change in fluid viscosity caused for instance by an about 5.degree. C. to 15.degree. C. decrease in temperature could result in a decrease in image quality, poor or unacceptable image development and undesirable image background development, for example, because of a 5 percent to 23 percent decrease in electrophoretic mobility. Insufficient particle charge can also result in poor, or no transfer of the developer or toner to paper, or other substrates. Poor transfer, for example, can result in poor image solid area coverage if insufficient toner is transferred to the final substrate and can also result in image defects such as smearing and hollowed fine features. To overcome or minimize such problems, the liquid toners of the present invention were arrived at after substantial research efforts, and which toners result in, for example, sufficient particle charge, generally corresponding to an ESA mobility greater than +2.0 E-10 m.sup.2 /Vs for excellent transfer and maintaining the mobility within the desired range of the particular imaging system employed. Advantages associated with the present invention include improvements in the desired positive charge on the developer particles; in some instances the improvement, as measured by ESA mobility, is from +0.6 E-10 m.sup.2 /Vs without the charge director mixtures of this invention to +3.4 E-10 m.sup.2 /Vs when the charge director mixtures of the present invention are selected. The greater toner charge results in, for example, improved image development and higher quality images, such as higher resolutions with less background deposits.
A latent electrostatic image can be developed with toner particles dispersed in an insulating nonpolar liquid. The aforementioned dispersed materials are known as liquid toners or liquid developers. A latent electrostatic image may be generated by providing a photoconductive layer with a uniform electrostatic charge and subsequently discharging the electrostatic charge by exposing it to a modulated beam of radiant energy. Other methods are also known for forming latent electrostatic images such as, for example, providing a carrier with a dielectric surface and transferring a preformed electrostatic charge to the surface. After the latent image has been formed, the image is developed by colored toner particles dispersed in a nonpolar liquid. The image may then be transferred to a receiver sheet. Also known are ionographic imaging systems.
Typical liquid developers can comprise a thermoplastic resin, optional pigment, and a dispersant nonpolar liquid. Generally, a suitable colorant, such as a dye or pigment, is also present in the developer. The colored toner particles are dispersed in a nonpolar liquid which generally has a high volume resistivity in excess of 10.sup.9 ohm-centimeters, a low dielectric constant, for example below 3.0, and a high vapor pressure. Generally, the toner particles are less than 10 .mu.m (microns) average by area size as measured with the Horiba 700 Particle Sizer.
Since the formation of proper images depends primarily on the difference of the charge between the toner particles in the liquid developer and the latent electrostatic image to be developed, it is desirable to add a charge director compound and charge adjuvants which increase the magnitude of the charge, such as polyhydroxy compounds, amino alcohols, polybutylene succinimide compounds, aromatic hydrocarbons, metallic soaps, and the like, to the liquid developer comprising the thermoplastic resin, the nonpolar liquid and the colorant. A charge director can be of importance in controlling the charging properties of the toner to enable excellent quality images.
In U.S. Pat. No 5,035,972, the disclosure of which is totally incorporated herein by reference, there are illustrated liquid developers with quaternized ammonium AB diblock copolymer charge directors, and wherein the nitrogen in the ionic A block is quaternized with an alkylating agent.
U.S. Pat. No. 5,019,477, the disclosure of which is hereby totally incorporated by reference, discloses a liquid electrostatic developer comprising a nonpolar liquid, thermolplastic resin particles, and a charge director. The ionic or zwitterionic charge directors may include both negative charge directors such as lecithin, oil-soluble petroleum sulfonate and alkyl succinimide, and positive charge directors such as cobalt and iron naphthenates. The thermolplastic resin particles can comprise a mixture of (1) a polyethylene homolpolymer or a copolymer of (i) polyethylene and (ii) acrylic acid, methacrylic acid or alkyl esters thereof, wherein (ii) comprises 0.1 to 20 weight percent of the copolymer; and (2) a random copolymer of (iii) selected from the group consisting of vinyl toluene and styrene and (iv) selected from the group consisting of butadiene and acrylate. As the copolymer of polyethylene and methacrylic acid or methacrylic acid alkyl esters, NUCREL.RTM. may be selected.
U.S. Pat. No. 5,030,535 discloses a liquid developer composition comprising a liquid vehicle, a charge control additive and toner particles. The toner particles may contain pigment particles and a resin selected from the group consisting of polyolefins, halogenated polyolefins and mixtures thereof. The aforementioned liquid developers can be prepared by first dissolving the polymer resin in a liquid vehicle by heating at temperatures of from about 80.degree. C. to 120.degree. C., adding pigment to the hot polymer solution and attriting the mixture, and then cooling the mixture so that the polymer becomes insoluble in the liquid vehicle, thus forming an insoluble resin layer around the pigment particles.
U.S. Pat. No. 5,026,621 discloses a toner for electrophotography which comprises as main components a coloring component and a binder resin which is a block copolymer comprising a functional segment (A) consisting of at least one of a fluoroalkylacryl ester block unit or a fluoroalkyl methacryl ester block unit, and a compatible segment (B) consisting of a fluorine-free vinyl or olefin monomer block unit. The functional segment of the block copolymer is oriented to the surface of the block polymer, and the compatible segment thereof is oriented to be compatible with other resins and a coloring agent contained in the toner so that the toner is provided with both liquid-repelling and solvent-soluble properties.
In U.S. Pat. No. 4,707,429 there are illustrated, for example, liquid developers with an aluminum stearate charge adjuvant. Liquid developers with, for example, certain aluminum salicylates as charge directors are illustrated in U.S. Pat. No. 5,045,425. Also, stain elimination in consecutive colored liquid toners is illustrated in U.S. Pat. No. 5,069,995.
In U.S. Pat. Nos. 5,306,591 and 5,308,731, the disclosures of which are totally incorporated herein by reference, there is illustrated a liquid developer comprised of thermoplastic resin particles, a charge director, and a charge adjuvant comprised of an imine bisquinone; and a liquid developer comprised of a liquid, thermoplastic resin particles, a nonpolar liquid soluble charge director, and a charge adjuvant comprised of a metal hydroxycarboxylic acid, respectively. In United States Statutory Invention Registration No. H1483, the disclosure of which is totally incorporated herein by reference, there is illustrated a liquid developer comprised of thermoplastic resin particles, and a charge director comprised of an ammonium AB diblock copolymer of the formula ##STR3## wherein X- is a conjugate base or anion of a strong acid; R is hydrogen or alkyl; R' is alkyl; R" is an alkyl group containing from about 6 to about 20 carbon atoms; and y and x represent the number average degree of polymerization (DP) wherein the ratio of y to x is in the range of from about 10 to 2 to about 100 to 20. The charge adjuvants and other appropriate components of these copending applications may be selected for the liquid toners of the present invention.
In U.S. Pat. No. 5,366,840, the disclosure of which is totally incorporated herein by reference, there is illustrated a liquid developer comprised of thermoplastic resin particles, an optional charge director, and a charge additive or adjuvant, including ALOHOS, comprised of a component of the formulas ##STR4## wherein R.sub.1 is selected from the group consisting of hydrogen and alkyl, and n is 0 (zero), 1, 2, 3, or 4.
Illustrated in U.S. Pat. No. 5,409,796, the disclosure of which is totally incorporated herein by reference, is a positively charged liquid developer comprised of thermoplastic resin particles, optional pigment, a charge director, and a charge adjuvant comprised of a copolymer of an alkene and unsaturated acid derivative and wherein the acid derivative contains pendant ammonium groups, and wherein the charge adjuvant is associated with or combined with said resin and said optional pigment; and in U.S. Pat. No. 5,411,834, the disclosure of which is totally incorporated herein by reference, and filed concurrently herewith, is a negatively charged liquid developer comprised of thermoplastic resin particles, optional pigment, a charge director, and an insoluble charge adjuvant comprised of a copolymer of an alkene and unsaturated acid derivative and wherein the acid derivative contains pendant fluoroalkyl or pendant fluoroaryl groups, and wherein the charge adjuvant is associated with or combined with said resin and said optional pigment.
In copending patent application U.S. Ser. No. 986,316, the disclosure of which is totally incorporated herein by reference, there is illustrated a process for forming images which comprises (a) generating an electrostatic latent image; (b) contacting the latent image with a developer comprising a colorant and a substantial amount of a vehicle with a melting point of at least about 25.degree. C., said developer having a melting point of at least about 25.degree. C., the contact occurring while the developer is maintained at a temperature at or above its melting point, the developer having a viscosity of no more than about 500 centipoise and a resistivity of no less than about 10.sup.8 ohm-cm at the temperature maintained while the developer is in contact with the latent image; and (c) cooling the developed image to a temperature below its melting point subsequent to development.